Servo controlled valves for controlling pressure or pressure differential are important in many hydraulic servo systems and have a large number of potential uses in fuel metering, fluid metering, and other technical fields. Servo valves for this purpose commonly have been rather complicated, underdamped and therefore prone to oscillation, and expensive. Current valves also have significant problems with stiction and other hysteretic characteristics which greatly complicate the design and accuracy of design of the servo mechanisms of which they form a part.
It is the purpose of the present invention to provide a pressure differential controlling servo valve where the pressure differential across the valve is a simple and smooth function of current, where the valve has no hysteretic or stiction characteristics, and where the speed of the valve is extremely fast (tau 1/e response on the order of a millisecond). It is a further purpose of the present invention to design such a servo valve having strong damping characteristics which can readily be designed to make servo system as a whole critically damped, or set at any other desired damping ratio. This damping feature will make it convenient for the servo designer to achieve stability and simplicity in his servo design.
The servo valve of the present invention is built on the basis of well-known laminar fluid mechanics, taking advantage of the very low magnetic hysteresis and rapid response characteristics of ferrite materials. A ferrite magnet constructed geometrically in a manner very similar to that of pot cores which are used conventionally in many high frequency circuits exerts a magnetic force on a floating plate which responds instantaneously to coil current variation and this magnetic force is resisted by an equal and opposite pressure force caused by the pressure differential across the plate so that the plate floats on a film or fuel or other liquid in the manner of the hydrostatic bearing (the characteristics of which can be exactly calculated). Changes in the film thickness separating the plate from the ferrite magnet rapidly change liquid flow across the valve. Film thickness changes are damped by the well-known squeeze film effect, and by proper choice of dimensions and radius ratios it is possible to control the squeeze film damping of the plate over a very wide range to control the damping characteristics of the valve in a manner convenient to the servo designer. The servo valve is a magnetically actuated hydrostatic bearing with squeeze film damping. As such, the servo valve can be built with very rapid and analytically predictable response, exactly calculable damping, and zero mechanical hysteresis and stiction. The valve is inexpensive to make and durable so long as magnetic particles are carefully filtered from the fluid flowing past the servo valve. These desirable characteristics are achieved in detail as follows.